Load transfer mechanism of fully grouted cable bolts.

Abstract

Fully grouted cable bolts have been used in the mining industry for more than fifty years. However, failure of cable bolting systems still occurs in mine sites, as a result of insufficient load transfer between the rock mass and the cable bolt. Numerous research studies have been conducted to understand the load transfer mechanism of cable bolts in the laboratory. However, there are many shortcomings in previous methods. This thesis aims to propose a new approach to study the load transfer mechanism of cable bolts. As part of this research, a modified Laboratory Short Encapsulation Pull Test (LSEPT) facility was developed. It was mainly composed of two parts, namely the embedment section and the anchor section. The influence of termination methods, borehole roughness and pre-confinement on the performance of cable bolts was investigated. The results show that the pre-confinement has a significant effect on the performance of cable bolts. The load transfer behaviour of two types of cable bolts, the Superstrand cable and the MW9 cable, was studied. The influence of different parameters including the cable surface geometry, borehole size and confining medium strength was evaluated. The results show that bond failure of Superstrand cable bolting systems always occurred at the cable/grout interface. However, for the MW9 cable installed in a standard borehole, bond failure occurred at the grout/rock interface in the weak confining medium. An analytical model was used to study the load transfer behaviour of cable bolts. Experimental results were adopted to validate this model. It was found that the cable/grout interface undergoes different stages in a pull-out test and the non-uniform shear stress distribution is more apparent in long embedment length.

The shear performance of the Stratabinder grout in the constant normal stress and constant normal stiffness conditions was also studied. The results reveal that the low normal stiffness has little impact on the performance of the grout. FLAC2D software package was used to simulate the axial behaviour of cable bolts. The calibration process of samples and the pull-out performance of cable bolts were validated with experimental results. It was found that the modified surface geometry has an obvious influence on inducing more confining pressure in pull-out tests.